1. Field of the Invention
This invention relates to machine tools and is particularly directed to machine tools having jaws for gripping a workpiece with means for sensing the position of the jaw and for causing outward movement of the jaw as the jaw tightens on the workpiece to prevent buckling of the workpiece.
2. Prior Art
In the course of metal part forming, such as straightening, detwisting, stretch wrap forming and the like, the workpiece is usually gripped in the jaws of a machine tool, in order to subject the workpiece to a desired thermal or mechanical process. Many of the machine tool jaws, used for gripping workpieces, are merely large versions of the familiar chuck or collet found in hand drills and constructed with a hollow housing having interior walls which taper inwardly toward the gripping end of the housing. A plurality of jaw members are located with the housing and are formed with inclined outer surfaces and means are provided for moving the jaw members toward or away from the gripping end of the housing. As the jaw members are moved toward the gripping end of the housing, the inclined outer surfaces of the jaw members bear against the tapered inner walls and are forced to move radially inwardly toward each other and to grip any workpiece inserted between the jaw members. Once the workpiece is engaged by the jaw members, any attempt to draw the workpiece outwardly from the housing will cause the jaw members to be pulled toward the gripping end of the housing and, hence, will cause the jaw members to further tighten their grip on the workpiece.
When the workpiece is to be held between two opposing chucks or collets, as in a lathe or the like, it is, first, customary to securely grip one end of the workpiece in one of the collets, which is usually fixed in position along the bed of the machine tool. Thereafter, the jaw members of the second collet are fully opened to freely accept the workpiece. After the workpiece is inserted between the jaws of the second collet, the jaws of the second collet are caused to grip the workpiece by moving the jaw members toward the gripping end of its housing, either manually, by means of a chuck key or the like, or by suitable power-driven means. However, once the jaw members make contact with the workpiece, the continued movement of the jaw members, which is necessary to make the jaw members firmly grip the workpiece, also causes the jaw members to push the workpiece outwardly of the gripping end of the housing. This pushing movement serves to drive the workpiece toward the first collet, which is stationary, and can be as much as 0.5 to 3.0 inches. Unfortunately, such movement often results in bending or buckling of the workpiece, which can damage or ruin expensive workpiece. This is a serious and expensive problem, even with highly-skilled and careful workmen. Nevertheless, virtually no prior art devices have been proposed to overcome this problem.
A search in the United States Patent Office has revealed the following references:
______________________________________ U.S. PAT. NO. INVENTOR ISSUED ______________________________________ 4,989,887 P. T. Jordan Feb. 5, 1991 4,971,339 C. C. Treff Nov. 2.0., 199.0. 4,938,49.0. R. P. Bosek Jul. 3, 199.0. 4,69.0.,415 W. B. Holdridge Sep. 1, 1987 ______________________________________
The patents to Jordan, Treff and Bosek each relate to means for ensuring that the jaw members move radially in a controlled or uniform manner to provide uniform radial gripping and positioning of the workpiece. However, none of these patents concerns axial movement of the workpiece. The patent to Holdridge is directed to preventing axial movement of the workpiece. However, the Holdridge mechanism is complex and expensive to install and requires considerable ongoing maintenance. Thus, none of the prior art devices are entirely satisfactory.